Motor control system and method

ABSTRACT

A method and apparatus for catching a rotating motor is described. Switches (typically the lower switches) of an inverter of a motor drive circuit are short-circuited for a brief period of time. The pulses generated by this process are analysed to determine the speed and direction of rotation of the motor. The pulses are checked to determine whether they are orthogonal. If they are, the motor velocity determination is deemed to be reliable and then motor is magnetised and normal motor control can be resumed. If not, the motor velocity determination is deemed to be unreliable and a magnetization pulse before the velocity determination step is repeated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of International PatentApplication No. PCT/EP2017/050161, filed on Jan. 4, 2017, which claimspriority to U.S. Patent Application No. 62/275,872, filed on Jan. 7,2016, each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a motor control system and method.

BACKGROUND

Adjustable speed drives are known for use in the control of multi-phaseelectric motors. In some cases, the state of an electric motor beingdriven by an adjustable speed drive may be unknown. Specifically, theangular position and/or the speed of rotation of a rotor of the motormay be unknown. This can happen for many different reasons. Such asituation can occur, for example, following a short power outage.Alternatively, this situation can arrive when a rotor is alreadyspinning before motor drive control has commenced (i.e. on startup ofthe motor drive).

Thus, a motor may have both mechanical and electrical energy storedtherein before being under the control of a motor drive. Algorithms areknown that seek to enable a motor drive to take control of a motorwithout creating a large disturbance in the energy stored in the machineand to do so without taking all the energy out of the motor. However,there remains a need for improved and alternative motor control systemsand methods.

The present invention seeks to provide an alternative method to thosealready available.

SUMMARY

The present invention provides a method (of catching or controlling amotor) comprising: estimating a frequency of rotation of a motor (i.e.motor speed) by repeatedly short-circuiting at least some windings ofthe motor in order to generate a plurality of short circuit currentpulses, identifying the peaks of the short circuit current pulses andusing the identified peaks to estimate said frequency (and typicallyalso direction) of rotation; determining whether the estimation of thefrequency of rotation of the motor is reliable; and in the event thatthe estimation of the frequency of rotation of the motor is determinedto be unreliable, applying a magnetising pulse to the motor andrepeating the steps of estimating the frequency of rotation of the motorand determining whether the estimation of the frequency of rotation ofthe motor is reliable.

The present invention further provides a controller (e.g. a motorcontroller/flystart module) comprising: a first input for receiving aplurality of short-circuit current pulses from at least some windings ofthe a motor; a control module configured to identify peaks of the shortcircuit current pulses and using the identified peaks to estimate saidfrequency (and typically also direction) of rotation of the motor, thecontrol module further configured to: determine whether the estimationof the frequency of rotation of the motor is reliable; and in the eventthat the estimation of the frequency of rotation of the motor isdetermined to be unreliable, to instruct the application of amagnetizing pulse to the motor and to repeat the steps of estimating thefrequency of rotation of the motor and determining whether theestimation of the frequency of rotation of the motor is reliable. Thecontroller may further comprise a frequency estimator (e.g.incorporating a phase locked loop) configured to generate the estimateof the frequency (and direction) of rotation of the motor. The frequencyestimator may be further configured to determine whether signalsindicative of cosine and sine vectors of measured currents areorthogonal.

Determining whether the estimation of the frequency of rotation of themotor is reliable may comprise determining whether signals indicative ofcosine and sine vectors of measured currents are orthogonal. Morespecifically, determining whether the signals indicative of the cosineand sine vectors are orthogonal may include determining whether a dotproduct of the cosine and sine vectors is below a threshold (e.g. closeto zero). In one form of the invention, the cosine and sine vectors arethe Clarke's transform of the peaks of the short circuit current pulses.

The invention may further comprise determining that the motor is atstandstill in the event that repeating the step of determining whetherthe estimation of the frequency of rotation of the motor is reliableresults in a determination that the frequency of rotation of the motorestimate is still unreliable.

In some forms of the invention, applying a magnetising pulse to themotor comprises injecting a ramping multi-phase (typically 3-phase)current into the motor (typically at a predetermined rate). This istypically done until the rated current of the machine is reached. Acurrent regulator may be used to generate the magnetization pulse.

Some forms of the invention include increasing the magnetizing levelwithin the motor (and then exiting the method), if the estimation of thefrequency of rotation of the motor is determined to be reliable. Thestep of increasing the magnetizing level within the motor may compriseinjecting a multi-phase (typically three phase) current into the motor(e.g. until the motor is fully magnetised).

The present invention yet further provides a motor drive circuitcomprising a controller as set out above and further comprising aninverter under the control of said controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference tothe following schematic drawings, in which:

FIG. 1 shows a multi-phase motor drive system;

FIG. 2 shows an inverter that may be used in the motor drive system ofFIG. 1;

FIG. 3 is a flow chart of an algorithm in accordance with an aspect ofthe present invention;

FIG. 4 shows current waveforms on motor windings in accordance with anaspect of the present invention;

FIG. 5 shows envelopes of current waveforms in accordance with an aspectof the present invention;

FIG. 6 shows estimated frequency over time in accordance with an aspectof the present invention;

FIG. 7 shows a phase-locked-loop circuit in accordance with an aspect ofthe present invention;

FIG. 8 shows magnetization pulses in accordance with an aspect of thepresent invention;

FIG. 9 shows magnetization ramp signals in accordance with an aspect ofthe present invention;

FIG. 10 shows signals of a motor starting arrangement in accordance withan aspect of the present invention;

FIG. 11 shows signals of a motor starting arrangement in accordance witha further aspect of the present invention; and

FIG. 12 shows a block diagram of a system in accordance with an aspectof the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system, indicated generally by thereference numeral 1, including an adjustable speed drive (ASD). Thesystem 1 comprises an AC power supply 2, an ASD 4 and a load 6 (such asa three-phase motor). The ASD 4 includes a rectifier 8 (often adiode-based rectifier, as shown in FIG. 1, although alternatives, suchas advanced front end rectifiers are known), a DC link capacitor 10, aninverter 12 and a control module 14.

The output of the AC power source 2 is connected to the input of therectifier 8. The output of the rectifier 8 provides DC power to theinverter 12. As described further below, the inverter 12 includes aswitching module used to convert the DC voltage into an AC voltagehaving a frequency and phase dependent on gate control signals. The gatecontrol signals are typically provided by the control module 14. In thisway, the frequency and phase of each input to the load 6 can becontrolled.

The inverter 12 is typically in two-way communication with the controlmodule 14. The inverter 12 may monitor currents and voltages in each ofthe three connections to the load 6 (assuming a three-phase load isbeing driven) and may provide current and voltage data to the controlmodule 14 (although the use of both current and voltage sensors is by nomeans essential). The control module 14 may make use of the currentand/or voltage data (where available) when generating the gate controlsignals required to operate the load as desired; another arrangement isto estimate the currents from the drawn voltages and the switchingpatterns—other control arrangements also exist.

FIG. 2 shows details of an exemplary implementation of the inverter 12.

As shown in FIG. 2, the inverter 12 comprises first, second and thirdhigh-sided switching elements (T1, T2 and T3) and first, second andthird low-sided switching elements (T4, T5 and T6). Each switchingelement may, for example, be an insulated-gate bipolar transistor (IGBT)or a MOSFET transistor. As shown in FIG. 2, each of the switchingelements (T1 to T6) is associated with a corresponding free-wheelingdiode (D1 to D6).

The exemplary inverter 12 is a three-phase inverter generating threeoutputs: U, V and W. The three phases of the inverter 12 provide inputsto the three-phases of the load 6 in the system 1 described above. Ofcourse, the inverter 12 could be modified to provide a different numberof outputs in order to drive a different load (such as a load with moreor fewer than three phases).

The first high-sided switching element T1 and the first low-sidedswitching element T4 are connected together between the positive andnegative DC terminals. The mid-point of those switching elementsprovides the U-phase output. In a similar manner, the second high-sidedswitching element T2 and the second low-sided switching element T5 areconnected together between the positive and negative DC terminals withthe mid-point of those switching elements providing the V-phase output.Furthermore, the third high-sided switching element T3 and the thirdlow-sided switching element T6 are connected together between thepositive and negative DC terminals with the mid-point of those switchingelements providing the W-phase output.

The inverter 12 is a 2-level, 6 transistor inverter. As will be apparentto those skilled in the art, the principles of the present invention areapplicable to different inverters, such as 3-level inverters. Thedescription of the inverter 12 is provided by way of example to helpillustrate the principles of the present invention.

FIG. 3 is a flow chart of an algorithm, indicated generally by thereference numeral 20, in accordance with an aspect of the presentinvention.

The algorithm 20 starts at step 22, during which step, initializationand calibration may occur, if required. The algorithm then moves toshorted windings estimation step (step 24).

At step 24, a number of switches of the inverter 12 are closed for abrief period of time in order to generate short-circuit pulses. In oneform of the invention, each of the low-side switching elements (T4, T5and T6 in the exemplary inverter 12) is closed periodically. In general,the short-circuiting must be long enough to get a measurable currentthat can be used later in the algorithm, but short enough to keep theinverter in a discontinuous mode of operation.

FIG. 4 is a graph, indicated generally by the reference numeral 40,showing current waveforms on motor windings in response to the shortcircuiting of the low-side switching elements as described above. FIG. 4shows three pulses and the 3 phase current waveform looks like a set ofsawtooth waveforms whose envelope is 3 phase sinusoidal.

Taking the peaks of the sawtooth waveform allows for the envelope to beseparated and used for frequency estimation. FIG. 5 shows envelopes(indicated generally by the reference numeral 50) of current waveformsin accordance with an aspect of the present invention. The sawtoothwaveforms visible in FIG. 4 can be filtered out by sampling theshort-circuit currents when the switches are open, thus obtaining thepeak of the sawtooth waveform.

The frequencies of the envelopes shown in FIG. 5 indicate the frequencyof rotation of the respective phases of the motor 6. FIG. 6 is a graph,indicated generally by the reference numeral 60, showing estimatedfrequency over time in accordance with an aspect of the presentinvention. The frequency estimation is based on the envelopes shown inFIG. 5 and corresponds to the frequency of rotation of the motor 6. Inthe example of FIG. 6, the frequency estimate settles at about 325radians/second.

At this stage, the shorted windings estimation step 24 of the algorithm20 has been used to obtain an estimate of the speed of rotation of themotor.

As noted above, the frequencies of rotation of the envelopes shown inFIG. 5 are equal to the electrical frequency rotation of the motor. Theenvelopes are used as the input of a Phase Lock and Loop, PLL.

The idea behind the PLL is that the sine of the difference between theactual input vector angle θ and its estimate {circumflex over (θ)} canbe reduced to zero using a proportional-integral (PI) controller, thuslocking the detected phase to the actual angle. The estimated frequency{circumflex over (ω)} is then integrated to obtain the angle {circumflexover (θ)}. FIG. 7 shows the block diagram of a phase locked loop (PLL),indicated generally by the reference numeral 70, that may be used in thepresent invention.

As described above, the signals 50 shown in FIG. 5 are 3-phase currentsignals that represent the envelopes of the current signals detected inthe shorted windings step 24 of the algorithm 20. The three-phasecurrent signals 50 are converted into 2-phase signals i_(saHigh) andi_(sbHigh) using a Clarke's transformation. Thus, the signals i_(saHigh)and i_(sbHigh) that are used in the PLL 70 are the Clarke's transform ofthe peak currents of the sawtooth waveform.

In normal operation, the currents vectors i_(saHigh) and i_(sbHigh) areat right-angles to one another. The angle generated by joining the endsof the currents vectors together defines the vector angle signal θ. Bysimple mathematics, the cosine and sine of the vector angle θ are givenby:

${\cos (\theta)} = \frac{i_{saHigh}}{\sqrt{i_{saHigh}^{2} + i_{sbHigh}^{2}}}$and${\sin (\theta)} = \frac{i_{sbHigh}}{\sqrt{i_{saHigh}^{2} + i_{sbHigh}^{2}}}$

In the phase locked loop circuit 70, a first function block 72 convertsthe current vectors i_(saHigh) and i_(sbHigh) into an estimate of cos(θ)using the formula:

${\cos (\theta)} = \frac{i_{saHigh}}{\sqrt{i_{saHigh}^{2} + i_{sbHigh}^{2}}}$

Similarly, a second function block 74 converts the current vectorsi_(saHigh) and i_(sbHigh) into an estimate of sin(θ) using the formula:

${\sin (\theta)} = \frac{i_{sbHigh}}{\sqrt{i_{saHigh}^{2} + i_{sbHigh}^{2}}}$

The PLL 70 generates an estimate of the vector angle. That estimate isdenoted by the symbol {circumflex over (θ)}.

As shown in FIG. 7, the estimate of cos(θ) is multiplied by the sine ofthe estimated vector angle to give: cos(θ)sin({circumflex over (θ)}).Similarly, the estimate of sin(θ) is multiplied by the cosine of theestimated vector angle to give: cos(θ)sin({circumflex over (θ)}).

An error term is calculated as follows:

ePLL=sin(θ−{circumflex over (θ)})=cos(θ)sin({circumflex over(θ)})−sin(θ)cos({circumflex over (θ)})   (1)

A PI controller is used such that the error term (ePLL) is forced tozero. In this way, signals for estimated frequency {circumflex over (ω)}and estimated vector angle signal {circumflex over (θ)} are obtained asoutputs of the PLL 70.

Returning to the algorithm 20 of FIG. 3, as described above, the shortedwindings estimation step 24 is used to give an estimated frequency,{circumflex over (ω)}. The algorithm 20 then moves to step 26, where anorthogonality test is used to determine whether the estimations made instep 24 are likely to be reliable. The orthogonality test step 26determines whether the signals i_(saHigh) and i_(sbHigh) are, in fact,orthogonal.

When the inputs of the PLL are extremely small and contain mostly noise(i.e. having a low Signal to Noise Ratio (SNR)), the cos(θ) and sin(θ)estimates will lose orthogonality. By checking for orthogonality of thecos(θ) and sin(θ) signals it is possible to determine whether the PLLestimation is valid or not. This is important because cos(θ) and sin(θ)must be orthogonal for the PLL to work properly.

The dot product of two vectors is given by:

Ā·B=|A||B|cos(Θ)   (2)

Since the cos(θ) and sin(θ) signals swing between ±1, the equationError! Reference source not found. can be reduced to:

Ā·B =cos(Θ)   (3)

If cos(θ) and sin(θ) are orthogonal the angle between them is 90 degrees(cos(90)=0) which means that when cos(θ) and sin(θ) are not corrupted:

Ā·B =0   (4)

Note this would have been true even if the magnitudes were not equal to1.

The dot product can be found iteratively by:

$\begin{matrix}{{\overset{\_}{A} \cdot \overset{\_}{B}} = {\sum\limits_{i = 1}^{n}{A_{i}B_{i}}}} & (5)\end{matrix}$

Thus, in step 26, the PLL uses the stationary reference frame currents(i_(saHigh) and i_(sbHigh)) which should be orthogonal to each other anduses a dot product as an orthogonality test to determine whether totrust the output of the PLL (determined when the dot product is below adetermined threshold i.e. close to zero). If the reference framecurrents are orthogonal, the algorithm 20 moves to step 34 describedfurther below; otherwise, the algorithm moves to step 28.

At step 28 of the algorithm 20, it has been determined that the shortedwinding estimation has not provided a reliable estimation of the rotorposition. This is likely to be because the level of magnetisation in themotor winding is insufficient for the short-circuit steps to providereliable data. Accordingly, at step 28, a magnetization pulse is appliedto the motor.

FIG. 8 shows a magnetization pulse, indicated generally by the referencenumeral 80, in accordance with an aspect of the present invention. Themagnetization pulse 80 injects a ramping three-phase current into themachine at a predetermined rate until the rated current of the machineis reached. A current regulator is used to create the current ramp. Thecurrent regulator requires a ramp rate and a target current value asinputs and outputs the three phase current ramp, such as that shown inFIG. 8.

When a current ramp is applied to a rotating induction machine the backemf (bemf) signal has two components:

-   -   A ramp    -   A sinusoid signal rotating at the frequency of the machine.

If the ramp is stopped before the ramp signal dominates the sinusoidsignal, it is possible to get a minima of the bemf. Catching a minima ofthe bemf means that the current signal seen during the short windingestimation will be too low to get any useful information.

Once the magnetization pulse is applied, the step 28 is complete. Thealgorithm 20 then moves to step 30.

Step 30 is a second “shorted windings estimation” step and repeats thestep 24 described above. Following, the application of the magnetizingpulse in step 28, the shorting process should result in correctmeasurements of rotor position.

From step 30, the algorithm 20 moves to step 32, where the orthogonalitytest (as described above with reference to step 26) is repeated. If thedot product is below the relevant threshold (i.e. near zero), thealgorithm 20 moves to step 34. If the dot product is still not below therelevant threshold, it is assumed that the motor is at (or very closeto) standstill and the algorithm 20 moves to step 36.

Step 34 of the algorithm 20 occurs when it has been determined thatrotor position estimations are reliable (either in step 26 or step 32 ofthe algorithm 20). At this stage, the frequency of rotation of eachphase of the motor has been deemed to have a reliable accuracy. Toensure that the magnetism level of the motor is at a suitable level(i.e. at a level such that the torque required to accelerate ordecelerate the motor is available), the step 34 increases the flux inthe machine to the appropriate level. When the flux is at theappropriate level, the algorithm 20 moves to step 36.

The step 34 is a “ramp magnetization level” step. The step 34 injectsthree-phase current into the motor 6 until the machine is fullymagnetized. FIG. 9 shows an exemplary magnetization ramp signals,indicated generally by the reference numeral 90.

Since the speed of rotation of the motor has been determined, the phasecurrents applied in step 34 should match the already spinning motor. Ifthis occurs, the motor is said to be “caught”. It should be noted thatif the estimation of the speed of rotation of the motor is incorrect bya small amount, then the increasing currents applied during the rampstep 34 will force the motor to follow the applied currents. Thus,despite small estimation errors, the motor is still “caught”. This ispossible if the error is sufficiently small and has been found to workwell in practice.

At step 36 of the algorithm 20, either the motor has be “caught” at step24 or step 28, or it is determined that the motor is at standstill (atstep 32). In any event, the motor velocity is known and so normalcontrol can commence. The algorithm 36 can therefore exit and normalmotor control applied.

FIG. 10 shows signals, indicated generally by the reference numeral 100,of a motor starting arrangement in accordance with an aspect of thepresent invention, thereby demonstrating an application of the algorithm20. In the example of FIG. 10, the motor 6 being started is initiallydemagnetized.

The algorithm 20 starts within the initialisation step 22 followed bythe shorted winding estimation step 24. The signals at this part of thealgorithm are indicated generally by the reference numeral 102. Sincethe motor 6 is demagnetized, the short-circuit pulses applied in step 24of the algorithm 20 do not result is current signal being generated.Accordingly, the portion 102 of the signals 100 do not show any currentpulses.

Since the short circuit pulses at step 24 do not lead to currents beinggenerated, the orthogonal test (step 26) fails and the algorithm movesto step 28 where a magnetizing pulse is applied (step 28). Themagnetizing pulses are clearly visible at part 104 of the signals 100.

With the magnetizing pulse applied, the algorithm 20 moves to step 30where short circuit pulses are again applied—this is shown at part 106of the signals 100. Although it appears that no current is generated inresponse to the short-circuiting of the windings, in fact currents aregenerated, but they are too small to be visible in FIG. 10.

Following the step 30, the orthogonality of the current pulses ischecked (step 32). This should now return a positive result, so that thealgorithm 20 moves to step 34. At step 34, the magnetization level isramped up, as shown in part 108 of the signals 100.

FIG. 11 shows signals, indicated generally by the reference numeral 110,of a motor starting arrangement in accordance with a further aspect ofthe present invention, thereby demonstrating a further application ofthe algorithm 20. In the example of FIG. 10, the motor 6 being startedis initially magnetized.

The algorithm 20 starts within the initialisation step 22 followed bythe shorted winding estimation step 24. The signals at this part of thealgorithm are indicated generally by the reference numeral 112. Sincethe motor 6 is magnetized, the short-circuit pulses applied in step 24of the algorithm 20 result is current signals being generated.Accordingly, the part 112 shows small current pulses.

Since short-circuit current pulses are generated from the magnetizedmotor, the orthogonal test (step 26) indicates that the current pulsesare orthogonal and so the algorithm 20 moves to step 34. At step 34, themagnetisation level is ramped up, as shown in part 114 of the signals100.

FIG. 12 shows a block diagram of a system, indicated generally by thereference numeral 120, in accordance with an aspect of the presentinvention.

The system 120 comprises an inverter 12 and a multi-phase motor 6 asdescribed above with reference to FIG. 1. The inverter has a DC input.The DC input may, of course, be obtained from an AC supply using arectifier in the manner described with reference to FIG. 1.

The inverter 12 in the system 120 is controlled using a switching logiccontroller 122. The switching logic controller 122 provides PWM signalsfor the inverter switches and may obtain control information (such asvoltages and/or currents) from the inverter 12. The switching logiccontroller 122 is in two-way communication with a frequency estimator126 and a current regulator 124. The frequency estimator 126 may providethe functionality of the PLL 70 described above. The current regulator124 may provide the control signals required to enable the inverter 12to generate the magnetization signals described above with reference toFIGS. 8 and/or 9.

The system 120 is highly schematic. Clearly, many variants are possible.

The embodiments of the invention described above are provided by way ofexample only. The skilled person will be aware of many modifications,changes and substitutions that could be made without departing from thescope of the present invention. The claims of the present invention areintended to cover all such modifications, changes and substitutions asfall within the spirit and scope of the invention.

What is claimed is:
 1. A method comprising: estimating a frequency ofrotation of a motor by repeatedly short-circuiting at least somewindings of the motor in order to generate a plurality of short circuitcurrent pulses, identifying the peaks of the short circuit currentpulses and using the identified peaks to estimate said frequency ofrotation; determining whether the estimation of the frequency ofrotation of the motor is reliable; and in the event that the estimationof the frequency of rotation of the motor is determined to beunreliable, applying a magnetising pulse to the motor and repeating thesteps of estimating the frequency of rotation of the motor anddetermining whether the estimation of the frequency of rotation of themotor is reliable.
 2. The method as claimed in claim 1, whereindetermining whether the estimation of the frequency of rotation of themotor is reliable comprises determining whether signals indicative ofcosine and sine vectors of measured currents are orthogonal.
 3. Themethod as claimed in claim 2, further comprising determining whether thesignals indicative of the cosine and sine vectors are orthogonal bydetermining whether a dot product of the cosine and sine vectors isbelow a threshold.
 4. The method as claimed in claim 1, furthercomprising determining that the motor is at standstill in the event thatrepeating the step of determining whether the estimation of thefrequency of rotation of the motor is reliable results in adetermination that the frequency of rotation of the motor estimate isstill unreliable.
 5. The method as claimed in claim 1, wherein applyinga magnetising pulse to the motor comprises injecting a rampingmulti-phase current into the motor.
 6. The method as claimed in claim 1,wherein, if the estimation of the frequency of rotation of the motor isdetermined to be reliable, increasing the magnetizing level within themotor.
 7. The method as claimed in claim 6, wherein increasing themagnetizing level within the motor comprises injecting a multi-phasecurrent into the motor.
 8. A controller comprising: a first input forreceiving a plurality of short-circuit current pulses from at least somewindings of the a motor; a control module configured to identify peaksof the short circuit current pulses and using the identified peaks toestimate said frequency of rotation of the motor, the control modulefurther configured to: determine whether the estimation of the frequencyof rotation of the motor is reliable; and in the event that theestimation of the frequency of rotation of the motor is determined to beunreliable, to instruct the application of a magnetizing pulse to themotor and to repeat the steps of estimating the frequency of rotation ofthe motor and determining whether the estimation of the frequency ofrotation of the motor is reliable.
 9. The controller as claimed in claim8, further comprising a frequency estimator configured to generate theestimate of the frequency of rotation of the motor.
 10. The controlleras claimed in claim 9, wherein the frequency estimator is furtherconfigured to determine whether signals indicative of cosine and sinevectors of measured currents are orthogonal.
 11. The controller asclaimed in claim 8, further comprising a current regulator to generatethe magnetization pulse.
 12. A motor drive circuit comprising acontroller as claimed in claim 8 and further comprising an inverterunder the control of said controller.
 13. The method as claimed in claim2, further comprising determining that the motor is at standstill in theevent that repeating the step of determining whether the estimation ofthe frequency of rotation of the motor is reliable results in adetermination that the frequency of rotation of the motor estimate isstill unreliable.
 14. The method as claimed in claim 3, furthercomprising determining that the motor is at standstill in the event thatrepeating the step of determining whether the estimation of thefrequency of rotation of the motor is reliable results in adetermination that the frequency of rotation of the motor estimate isstill unreliable.
 15. The method as claimed in claim 2, wherein applyinga magnetising pulse to the motor comprises injecting a rampingmulti-phase current into the motor.
 16. The method as claimed in claim3, wherein applying a magnetising pulse to the motor comprises injectinga ramping multi-phase current into the motor.
 17. The method as claimedin claim 4, wherein applying a magnetising pulse to the motor comprisesinjecting a ramping multi-phase current into the motor.
 18. The methodas claimed in claim 2, wherein, if the estimation of the frequency ofrotation of the motor is determined to be reliable, increasing themagnetizing level within the motor.
 19. The method as claimed in claim3, wherein, if the estimation of the frequency of rotation of the motoris determined to be reliable, increasing the magnetizing level withinthe motor.
 20. The method as claimed in claim 4, wherein, if theestimation of the frequency of rotation of the motor is determined to bereliable, increasing the magnetizing level within the motor.